I was hoping you folks could help me with a diagnosis issue.
I recently bought a 99 540i Station Wagon with 77K miles. The exhaust seems to be louder than it should when I accelerate. I had it put on a lift and the mechanic said that there was no exhaust leak at all. He suggested that I go to A BMW mechanic.

Here is the strange part of the problem. When it is raining an I go over a puddle the noise gets very loud. It is also directly proportionate to the size of the puddle. The bigger the puddle the louder the noise. The increase in noise only lasts a few minutes (I guess until the water evaporates. I am taking it in for service this week but I was hoping someone on this forum would have some insight.

Thats an interesting problem... honestly I dont see how water can make your exhaust louder... nor have I ever heard of such a thing... unless you have a huge leak that would allow the water to enter. As for the loudness... I'd probably say replace your exhaust if you want something quieter. Many stock BMW exhausts are loud to start with... to some people's ear.